1. Field of the Invention
The present invention relates to a vehicle air conditioner having a compressor, which is driven by a vehicle engine when a cooling load of a passenger compartment is higher than a predetermined value, and is driven by an electrical motor when the cooling load of the passenger compartment is lower than the predetermined value.
2. Description of Related Art
In a vehicle air conditioner described in U.S. Pat. No. 5,867,996 (corresponding to JP-A-10-236151), a variable displacement compressor is driven by a vehicle engine having a large output when a cooling load of a passenger compartment is larger than a predetermined value. Further, when the cooling load of the passenger compartment becomes lower than the predetermined value, the variable displacement compressor is driven by an electrical motor having an output smaller than that of the vehicle engine. However, according to experiments by the inventors of the present application, in a case where the cooling load is determined based on a deviation between an actual temperature of an evaporator and a target temperature of the evaporator, a necessary cooling capacity may be obtained even if the compressor is driven by the electrical motor when the cooling load is higher than the predetermined value. In this case, fuel consumption efficiency of the vehicle engine may be deteriorated.
In view of the above problem, it is an object of the present invention to improve fuel consumption efficiency of a vehicle engine while at least a minimum of necessary cooling capacity can be obtained, in a vehicle air conditioner having a compressor that is driven by the vehicle engine when the cooling load is higher than a predetermined value and is driven by an electrical motor when the cooling load is lower than the predetermined value.
According to a first aspect of the present invention, a vehicle air conditioner includes a compressor of a refrigerant cycle driven by at least one of an engine and an electrical motor for compressing refrigerant, and an evaporator for cooling air to be blown into a passenger compartment of the vehicle by performing a heat exchange between the air and low-pressure low-temperature refrigerant in the refrigerant cycle supplied by operation of the compressor, and an air-conditioning control unit for controlling the compressor. Further, the air-conditioning control unit includes a target temperature calculating means for calculating a target temperature of the evaporator to be lower as a cooling load in the passenger compartment becomes higher, a control means for controlling the compressor in such a manner that an actual temperature of the evaporator approaches to the target temperature, and a basis temperature setting means for setting a basis temperature that is higher than the target temperature by a predetermined temperature. When the actual temperature of the evaporator is higher than the basis temperature, the air-conditioning control unit outputs an engine drive signal for requiring a drive of the engine, and the compressor is operated by a driving force transmitted from the engine. On the other hand, when the actual temperature of the evaporator is lower than the basis temperature, the air-conditioning control unit prohibits an output of the engine drive signal and outputs a motor drive signal for requiring a drive of the electrical motor to drive the electrical motor, and the compressor is operated by a driving force from the electrical motor. In the air conditioner, the basis temperature setting means includes a predetermined temperature calculation means for calculating the predetermined temperature in accordance with the target temperature such that the predetermined temperature becomes higher as the target temperature becomes lower, and the basis temperature setting means sets the basis temperature to be not higher than a predetermined comfortable limit temperature. Accordingly, the fuel consumption efficiency of the engine can be effectively improved while a minimum of cooling capacity of the passenger compartment can be ensured.
For example, as the compressor, a variable displacement compressor capable of adjusting its displacement from an outside can be used. In this case, a temperature variation in the evaporator can be effectively reduced.
According to a second aspect of the present invention, in a vehicle air conditioner, when an actual temperature of the evaporator is higher than a predetermined comfortable limit temperature, the air-conditioning control unit outputs an engine drive signal for requiring a drive of the engine, and the compressor is operated by a driving force transmitted from the engine. On the other hand, when the actual temperature of the evaporator is lower than the predetermined comfortable limit temperature and is higher than a first basis temperature that is higher than a target temperature by a first predetermined temperature, the air-conditioning control unit prohibits an output of the engine drive signal and outputs a motor drive signal for requiring a drive of the electrical motor to drive the electrical motor, and the compressor is operated by a driving force transmitted from the electrical motor. Further, when the actual temperature of the evaporator is lower than the first basis temperature, the air-conditioning control unit prohibits the output of the engine drive signal and an output of the motor drive signal, and stops operation of the compressor. Accordingly, the fuel consumption efficiency of the engine effectively can be improved while at least the minimum of the cooling capacity of the passenger compartment can be obtained.
Specifically, the comfortable limit temperature is a highest temperature of a second basis temperature that is higher than the target temperature by a second predetermined temperature larger than the first predetermined temperature, and the air-conditioning control unit sets the second predetermined temperature to be higher as the target temperature becomes lower.